Acids and Bases Explained: Definitions, Differences, and Examples
Acids and bases are two of the most important classes of substances in chemistry. In simple terms, an acid is a substance that releases hydrogen ions (H⁺) when dissolved in water, while a base is a substance that accepts hydrogen ions or releases hydroxide ions (OH⁻). This single difference explains most of how they behave.
What is an acid?
An acid is a substance that donates hydrogen ions (protons) to another substance. When an acid such as hydrochloric acid (HCl) dissolves in water, it splits apart to produce H⁺ ions, which are responsible for the acid’s characteristic sour taste and its ability to react with metals and carbonates.
Common acids include:
- Hydrochloric acid (HCl), found in your stomach, where it aids digestion.
- Sulfuric acid (H₂SO₄), used in car batteries and industry.
- Citric acid, gives lemons and oranges their sharp taste.
What is a base?
A base is a substance that accepts hydrogen ions or produces hydroxide ions (OH⁻) in water. Bases that dissolve in water are called alkalis. They typically feel slippery and taste bitter, and they neutralize acids.
Common bases include sodium hydroxide (NaOH), used in soap-making, and sodium bicarbonate (baking soda), a mild base used in cooking.
The pH scale
Chemists measure how acidic or basic a solution is using the pH scale, which runs from 0 to 14:
- pH below 7, acidic (lower numbers are more strongly acidic)
- pH of 7, neutral (pure water)
- pH above 7, basic, or alkaline
Each step on the scale represents a tenfold change in acidity, so a solution of pH 3 is ten times more acidic than one of pH 4.
Neutralization: when acids meet bases
When an acid reacts with a base, they cancel each other out in a reaction called neutralization. The products are always a salt and water:
acid + base → salt + water
For example, hydrochloric acid reacts with sodium hydroxide to form sodium chloride (common table salt) and water. Neutralization is why an antacid tablet (a base) can relieve the discomfort of excess stomach acid.
Key takeaways
- Acids donate H⁺ ions; bases accept them or donate OH⁻ ions.
- The pH scale (0–14) measures acidity, with 7 being neutral.
- Acids and bases neutralize each other to make a salt and water.
Want precise definitions of the terms above? Browse the Chemistry Dictionary for entries like acid, base, and pH.